This invention relates generally to a method for determining subsurface gradient characteristics of articles and deals more particularly with a system for the quantitative determination of hardness at sub-surface depths in operational components such as a jet engine bearing parts. More specifically this disclosure deals with non-destructive measurement of the hardness existing at calibrated depths below the surface of a body.
In today's aircraft gas turbine engines, bearings operate at speeds in the range of 1.8 to 2.0 million dN where d equals the inner ring bore diameter of the bearing in millimeters and N equals the shaft revolutions per minute. Future generation ball and roller bearings are now in the design and testing stage for operation at 3 million dN and higher, where engine efficiency is exponentially improved. At higher N speeds, greatly increased centrifugal forces are imposed upon the bearing raceways by the orbiting rolling elements. These radial forces not only substantially reduce the margin of bearing capacity available for supporting operational thrust, radial and overturning moment loads of the engine but also produce hoop tension in the constraining bearing outer rings. Other significant hoop tension stresses develop in the inner ring due to exposure to its own centrifugal environment, while the usual heavy press fit of the inner ring on the shaft creates additive hoop stress. In the case of multiple concentric engine shafts both inner and outer rings of intermediate position bearings experience centrifugal hoop stress as they each rotate at their own rates. From whatever source, tensile stresses promote crack initiation and propagation.
As engines become larger and concentric shafts prevail, bearing diameter, the d factor, increases and the d N challenge agains becomes more severe.
Engine thrust to weight ratios and space restrictions are improved when rolling contact bearings are designed to serve structural functions such as to eliminate the need for housings, baffles and bulkheads. In this role many bearing designs now feature flanges, scallops, bolt-holes, oil passages, seal grooves, or other geometric anomalies which introduce unusual bending and stress raising characteristics. These circumstances also promote crack initiation and propagation in today's high hardness brittle bearing materials.
Extensive testing has proven that today's through-hardened steel main shaft ball and roller bearings with all these newly required features have not been able to perform reliably above the 2.0 million dN barrier. Minor rolling contact fatigue cracks which may typically occur in the raceways of the relatively brittle high hardness steel tend to propagate at an alarming rate as a result of the stress raising design or externally applied structural loads, sometimes proceeding to sudden and unexpected catastrophic failure. A materials change must be made.
Recent bearing material research has shown that case-hardened low carbon tool steel can provide both the tough cores for structural strength and "crack-stopper" requirements and the high hardness surfaces for best rolling contact fatigue, wear and penetration resistance. The usual alloys, molybdenum, manganese, chromium, vanadium, etc., are selectively added to provide the hardenability, hot-hardness, dimensional stability, carbon absorption and corrosion resistance as appropriate.
Low carbon M50 or matrix alloy steels have been selected for the new bearings and when combined with rather conventional carburizing processes have proven in full scale tests to serve these multiple requirements. It is doubtful however that these case-hardened parts dare be flown in aircraft until some reliable method such as described herein is adopted to assure the air-worthiness of the otherwise hidden hard case characteristics.
Carbon enrichment of the surface and near-surface volume of an article, and subsequent preferential hardening so as to provide a hardening gradient is old in the art, but accurate non-destructive measurement of property gradient from surface to core and the depth of the hardened zone, presents a new challenge. It becomes most important in critical production manufacture that means be provided to analyze the physical properties through the subsurface transition zone and into the core. Destructive sampling inspection procedures do not insure the integrity of every kindred part. It would be of great advantage over present methods to be able to carry out this analysis precisely on each final part by a non-destructive testing method to qualify it for service in any highly critical application.
A general object of the present invention, then, is to provide a system which will assure that the hardness character of the surface, subsurface, and core meet the standards referred to previously. More particularly in the jet engine bearing environment the depth to which case hardening is achieved must not be so thick as to permit cracks to progress and lead to relatively sudden failure of the part, nor may this case hardening be so shallow as to lead to early spalling and too frequent bearing replacement requirements. As a result of utilizing the method and means described herein, the suitability of the material at all levels, surface and below, is insured for the rigors of performance.